Method And Device For Measuring The Concentration Of Substances In Gaseous Or Fluid Media Through Optical Spectroscopy Using Broadband Light Sources

ABSTRACT

The invention relates to a method and to a device for using partially non-stabilized broadband light sources to accurately measure partially broadband-absorbing substances using referencing measuring cells. In order to create a low-cost, high-resolution, and at the same time fast spectrographic device for measuring concentrations of substances in fluid or gaseous media that is also suitable for harsh environments, the light radiated by the broadband light sources ( 1 ) through light guiding optical systems is fed through the measuring section of the self-referencing measuring cell ( 20, 30, 40 ) or only partially through a measuring cell ( 10 ) to a measurement detector (photoreceptor  11 ) and partially through a reference path (optical waveguide  8 ) to a reference detector (photoreceptor  15 ), and a mode coupler ( 5, 9, 14 ) is associated with each optical waveguide ( 2, 4, 7, 8 ) in order to homogenize the radiation characteristic of the broadband light sources ( 1 ), which varies over time and space.

The invention relates to a method of referencing in the optical absorption spectroscopy using broad band light sources for determining the concentration of substance in gaseous or fluid media through and to a device for measuring the concentration of substance in gaseous or fluid media within the measurement path of a measurement cell using absorption spectroscopy of light emitted from broad band light sources via light guiding optics. The device is used among other things for measuring carbon monoxide (NO), carbon dioxide (NO₂), suflur dioxide (SO₂), ozone (O₃), as well as components in fluid media and others, for combustion engines, especially in the online monitoring of diesel combustion engines, in environmental measurement technique, in medical technology, for instance for the measurement of respiratory air and others.

The determination of the concentration of substances using spectroscopic methods via broadband light sources and spectral selective detectors, such as spectrometer of filtered optical detectors, is well known. Further more it is state of the art, to guide spectral selective sources, such as Laser or filtered broadband light sources, via a measurement path to a filtered or unfiltered detector, to thereby characterize for instance gases or fluids. A logical conclusion of that is the utilization of already spectral limited LED-light sources with and without optical filters. LEDs are to understand here as broadband light sources, because in opposite to narrow line with sources (such as Laser) they emit a comparable broad frequency spectrum. The utilization of light guiding optics such as optical wave guides (LWL), for mechanical and thermal decoupling respectively for the spatial separation of the measurement place and the source and receiving unit is also well known in the sensor technology.

The basic measurement principle of the optical spectroscopy is based on the measurement of light extinction that has passed a measurement cell. The inference to a defined substance concentration in the measurement cell is therewith only an indirect method. A reliable measurement can be realized due to the usage of additional so called reference wavelength, whereby the spectral characteristic of the substance is utilized. These issues result for example in a measurement setup that is shown in FIG. 5 in the report of M. Degner and H. Ewald “Low cost sensor for online detection of harmful diesel combustion gases in UV-VIS region” [SPIE Photonics Europe 2006, Photonics in the Automobile 11, ISBN 0-8194-6254-3, FR Strasbourg, April 2006]. In addition to broad band light sources very often tuneable Laser sources are used in spectroscopy to reach high resolution. An absorption band of the requested substance is there scanned with the Laser line. Here the intensity of the detected light outside the absorption band is used as a reference for the intensity at the place of the absorption band, because the intensity is attenuated in the range of the substance absorption band.

The disadvantage here is, that on the one hand a high concentration resolution can be realized using the laser spectroscopy, on the other hand the number of detectable substances is limited due to the availability of an adequate Laser light sources at the required interaction wavelength of the substance. In addition such arrangements often are cost intensive, less robust and therewith not suitable for mass production in the field of sensors.

The implementation of broadband light sources in combination with spectrometers leads likewise to cost intensive and in addition not very sensitive measurement arrangements. In this case the emission spectrum of the light broad band light source is compared with the spectrum after the light pass through the measurement cell. Filtered broadband band light sources and especially LEDs are in opposite to that a more cost effective alternative. The general problem of devices with broadband light sources is the spectral and temporal changing of the light intensity respectively the emission characteristics, whereby the resolution and especially the maximum reachable accuracy is strongly limited. In addition high measurement times are required to reach that high resolution because of the resulting limited optical power density (except some very specified LEDs).

Therefore the invention is based on the problem to provide a low cost, high resolution and at the same time fast spectroscopic method for the determination of the concentration of substances in gaseous or fluid media as well as a device for implementation of the method that is, respectively that are, robust versus exterior influences.

The solution to this problem is obtained, according to the characterizing features of the method claim, in that the light emitted by the broadband light sources is guided partially through the measurement path of a self referencing measurement cell to a measurement detector, and only partially through a reference path to a reference detector, wherein the measurement path and the reference path are partially identical, and the influences of the emission characteristics of the broadband light sources and of the mode effect of the optical components are avoided by way of mode couplers in the light paths. According to the characterizing features of the device claim the solution to this problem is obtained in that the light emitted by the broadband light sources via light guiding optics is guided through the measurement path of a self referencing measurement cell, or only partially through a measurement cell to a measurement detector and partially via a through a reference path to a reference detector, and that for homogenizing the temporally and spatially varying emission characteristics of the broadband light sources a mode coupler is referenced to the light guide optics and to the light paths, respectively. The Mode couplers should be dimensioned in a way, that attenuation or scattering is as low as possible. There are typically cost effective spectral selective broadband light sources used, its light is guided via a light guiding system into the measurement path and is spectral selective evaluated.

One problem with commonly used beam splitters in the spectroscopy is the limited stability of the accuracy of the ratio of the resulting beams. This is caused by the temporal changing of the spatial fluctuation of the emitted light intensity. The currently splitting of the light is there depending from the actual, temporal changing inhomogeneous emission characteristic of the broadband light source. According to the invention the homogenisation of the emission characteristic is realized by the use of the mode couplers. The afterwards splitting of the light is therewith no more depending on the fluctuating intensity allocation of the light source. The irradiated light principally “forgets” where it comes from. In principle also scattering plates (milk glass, diffusor) can be used but the attenuation of light intensity would be quite high here. According to the invention the mode coupling is realized within the mode couplers inside the light guiding optics respectively in the light paths. Therefore optical wave guides are well suited because these mode couplers can be integrated there. There are different realisation possibilities such as using a long fibre, taper, more dimensional bending coupler. By using mode couplers an efficient optical component for homogenization of the emission characteristic is utilized.

According to the invention the resulting temporal fluctuations of the broadband light source, especially LEDs, are compensated by use of a well suited reference arrangement, that means: The disturbances due to the measurement arrangement and the surrounding are compensated according the target value. Thus the measurement certainty is much bigger and especially there through higher measurement accuracy/resolution is achieved.

Due to the utilization of the mode coupler, realized for instance as a ring coupler and the fibre coupler for light separation a source independent, robust and thus exact referencing is feasible to for instance measure gas concentration below 1 ppm at a path length of some centimeters at a measurement time of some milliseconds.

Furthermore the invention uses another embodiment utilizing a self referencing measurement arrangement to reach on that way the wanted accuracy. Also here the coupling and mode mixing of the separated light sources with the help of fibre optics are used to reach for all wavelengths preferably the same path through the optical measurement cell. Therewith disturbances within the cell e.g. on the absorption and reference wavelength are effecting in the same way. In opposite to the previous described measurement setup here there is no second receiving channel for referencing required. Instead of this the effective absorption path length of the measurement cell is changed (self referencing) and therewith the measurement signal is modulated in a defined manner as it can not be caused by disturbances. The due to the receiving part detected .signal sequence can adequate be demodulated. There through at least to signal are generated, that can be used for referencing of each single wavelength. The advantage of this arrangement is that the whole optical path outside the measurement cell is identical and it is only within the measurement due to the modulation modified. There through only one receiving unit is required, there is no need for two identical receivers. Thus receiving disturbances influence the reference and the measurement signal in the same way. The disadvantage here is the higher complexity of the measurement cell design.

In an example arrangement due to active switching or inclining or rotating of a small plate of glass within the measurement cell the path length through the detectable media can be varied and thus a reference due to the measurement volume can be realized. Through the possible usage of a concave mirror as a reflector also wavelength depending disturbances of dispersion are reduced.

In a further example arrangement of the self referencing measurement cell a part of the light is reflected directly to the receiver at a first mirror. A second path is transmitted and reflected to the receiver at a second mirror in dependency of the orientation of a rotor. This rotor is propelled for instance due to the flow of the measurement media and modulates therewith the effective path length. Also here there is an important advantage compared to conventional reference arrangements due to the referencing within the measurement cell that enables disturbances are acting on the reference and measurement path in the same way where through a strong suppression of disturbances is realized.

Especially for gaseous media a further kind of referencing can be realized, through converting the indirect measurement principle of the optical spectroscopy into an other direct sensor effect. Through an additional modulation of the measurement variable for instance through variation of pressure, the volume concentration of the gas is changed and therewith the detected extinction measurement values of the single sources. Is the variation of this additional physical effect for instance due to the use of conventional sensors (e.g. pressure) simultaneously detected, so the effect of the modulation can be used to reference the whole system very accurately, disturbances are suppressed by this way and the real concentration values at normal pressure are estimated. In many systems pressure changing is inert, thus this is a very simple and effective method for self referencing of a measurement cell.

As one example of the above described methods for referencing and for the realization of low cost, precise spectroscopic sensors, the exhaust gas sensor for combustion processes respectively engines on the basis of novel UV-LEDs is mentioned here. This sensor is also constructively thought and probed for the usage in extremely rough environment, such as the exhaust channel of a car (amongst others high temperature, vibration, chemical aggressive media).

The device for measuring of substance concentrations in gaseous or fluid media according the invention is explained in detail in the following with the help of embodiments shown in the drawings. It is shown:

FIG. 1: a principle presentation of the first embodiment,

FIG. 2: a principle presentation of the second and third embodiment,

FIG. 3: a principle longitudinal section through one self referencing measurement cell according to the second in FIG. 2 shown embodiment using a inclining small plate of glass and

FIG. 4: a principle longitudinal section through a second self referencing measurement cell according the second in FIG. 2 shown embodiment.

The in FIG. 1 shown first embodiment of a device for measurement of substance concentration in media includes 1 . . . n spectral selective broadband light sources 1, for instance LEDs, their light is guided via light wave guides 2 to a first fibre coupler 3, it is mixed and coupled to one light path there. One to the first coupler 3 connected wave guide 4 is via a mode coupler 5 connected to a second to a second fibre coupler 6 (e.g. 75/25). From this two further wave guides 7 and 8 start, the wave guide 7 leads via a mode coupler 9 to a measurement cell 10 that contains the measurement media, further to a first photo detector 11, to an A/D-converter 12 and than to a controller 13, e.g. to a μC or a DSP. The second wave guide 8 leads the light of the source via a mode coupler 14 to a second photo detector 15, an A/D-converter 16 and than to the mentioned controller. Through an adequate signal analysis e.g. referencing through compare of the both receiving channels, a fibre optical referencing is realized. Thus a very precise concentration measurement of the substances within the measurement path of the measurement cell 10 is realizable. The electronic control of the spectral selective light sources occurs via the mentioned controller via the controller path 18, this is advantages for a synchronous control and signal acquisition according high measurement rates.

According the invention the homogenization of the emission characteristics of the 1 . . . n spectral selective light sources 1 occurs through the utilization of the mode coupler 5, 9, 14. The following splitting of the light therewith is independent of the fluctuating intensity allocation of the light sources 1. The irradiated light quasi “forgot” where it comes from. Here the mode coupling is implemented via the mode coupler 5, 9, 14 within the optical wave guides 2, 4, 7, 8. Therefore there are different realization possibilities such as the use of long fibres, taper, more dimensional bend coupler and others are usable. With the mode coupler 5, 9, 14 an efficient optical component for homogenisation of the emission characteristics is used.

The fluctuations of the filtered spectral selective broadband light sources 1 . . . n, especially LEDs, are compensated with the help of the reference arrangement, consisting of the light path from the wave guide 8, with mode coupler 14 and the second photo receiver 15 as well as the associated A/D-converter 16 with adequate signal analyses in the controller 13. Disturbances from the measurement arrangement and the surrounding are therewith suppressed versus the target value. Therefore the measurement reliability is substantially higher and in particular it is the reason to achieve a better measurement precision and resolution. By using of mode couplers 5, 9, 14, implemented e.g. as ring-couplers, and the fibre-couplers 3, 6 for splitting the light, a robust and therewith exact referencing is feasible which is independent from the source, to enable for example the measurement of gas concentrations with a resolution of less than 1 ppm at an absorption path length of few centimetres at a measuring time of a few milliseconds.

Second embodiment shown in FIG. 2 includes in the same way as in first embodiment shown in FIG. 1 the 1 . . . n spectral selective broadband light sources 1, the optical wave guides 2, the fibre coupler 3, the mode coupler 5, the wave guides 4, 7, the measurement cell 20 including the measuring path, the photo detector 11, the A/D-converter 12 and the controller 13. In opposite to the first embodiment regarding FIG. 1 the fibre optical splitting of the light with the reference path is missing here.

In this case the measurement 20 cell is self referencing realized; therefore the light path for instance regarding the presentation in FIG. 3 respectively FIG. 4 or according the third embodiment is modulated. For the detection of the modulation regarding the last mentioned kind of modulation, the sensor 21 with the measurement connection 19 is implemented and sends in the same way as the photo detector measurement values to the controller. Within the controller an adequate demodulation of the measurement signals is realized, its results are used for the referencing.

In the second embodiment according FIG. 2 is thus a self referencing measurement cell arrangement planed to reach a source independency and thus the target resolution. It also uses the coupling within the fibre coupler 3 and the light mode mixing of the single light sources 11 utilizing fibre optics in the mode coupler 5 to reach nearly the same optical path for all wavelengths through the measurement cell 20. Therewith disturbances on for instance the absorption—and reference wavelength within the measurement cell 20 result in the same way.

In opposite to the first embodiment according FIG. 1 here is no light splitting and no second receiving optical channel required for referencing. Instead of this the effective absorption path length of the measurement cell is modified (self referencing) and therewith the measurement signal is modulated in a defined way, as can not be affected by disturbances. The using the receiving unit detected signal sequence can be adequate demodulated. There through at least two signals are generated that are utilized for the referencing of each single wavelength. The advantage of this arrangement is that the whole optical path outside the measurement cell 20 is identical and it is only within the measurement cell 20 modified due to the modulation. There through only one receiving unit is required, there is no need for two identical receivers. Thus receiving disturbances influence the reference and the measurement signal in the same way. The disadvantage here is the higher complexity of the measurement cell design 30, 40.

Especially for gaseous media a further kind of referencing can be realized, through converting the indirect measurement principle of the optical spectroscopy into an other direct sensor effect, by using a measurement cell as it is described in the application example 1. Through an additional modulation of the measurement variable for instance through variation of pressure, the volume concentration of the gas is changed and therewith the detected extinction measurement values of the single sources. Is the variation of this additional physical effect for instance due to the use of conventional sensors 21 (e.g. pressure) detected, so the effect of this modulation can be used to reference the whole system very accurately, disturbances are suppressed by this way and the real concentration values at normal pressure are estimated. In many systems pressure changing is inert, thus this is a very simple and effective method for self referencing of a measurement cell.

The further embodiment according FIG. 3 shows the measurement cell 30, where the target substances are included, with in- and outlets 31, 32 for the from the light sources 1 emitted light that is guided via the optical wave guide 4 and the mode coupler 5, the measurement cell 30, the optical wave guide 7 to the photo detector 11. Within the measurement cell 30 a glass plate respectively a small glass plate 33 is integrated, due to its inclining or rotating the effective wavelength through the measurement media within the measurement cell 30 is switched/varied. In position II of the little glass plate 33 the light passes along the path shown by the arrow 38. In the position I of the glass plate 33 the light moves along the arrows 39 also via the in- and outlets 35, 36. Thus the reference is realised by the interaction with the media in the measurement volume of the measurement cell 30. It is an advantage, that the light has passed on both ways the same optical components—in position I and II. Through the usage of a concave mirror 34 as a reflector at the right side of the shown measurement cell 30 (FIG. 3) wavelength depending disturbances of dispersion are minimized. Also a plane mirror could be used here; in this case the media bordering glass plate 37 would be exchanged by a convex lens.

In the embodiment according FIG. 4 of the self referencing measurement cell 40 one part of the light, that is coupled into the cell via the inlet 31 and collimated at the collimating lens 41 into direction 45 is reflected at the half transmission mirror 42. This reflected light passes again the collimating lens 41 and is guided to the outlet 32 and finally to the receiving unit 11, 12, 13. A second part of the irradiated light passes according the direction 46 the half transmission mirror 42 and reaches the second measurement volume II. In dependency of the rotor 44 orientation in the optical path, this mentioned part of the light is reflected at a full mirror 43 (in FIG. 4, right) and reaches according 47 the outlet 32 respectively the receiving unit 11, 12, 13.

The rotor 44 is propelled for instance by the flow of the measurement media that is passing the measurement cell 40 in the measurement volume II in the direction of the arrows 48. The blades of the rotor 44 are used to interrupt the light transmission and therewith it modulates the effective wavelength of the light respectively the interaction strength of the light with the media. Within the measurement volume I and II of the measurement cell 40 there is here the same measurement media included.

As one example of the above described methods for referencing and for the realization of low cost, highly precise spectroscopic sensors, the exhaust gas sensor for combustion processes respectively—engines on the basis of novel UV-LEDs is mentioned here. This sensor is also constructively thought and probed for the usage in extremely rough environment, such as the exhaust channel of a car (amongst others high temperature, vibration, chemical aggressive media).

LIST OF REFERENCE NUMBERS

-   1 Light source -   2 Optical wave guide -   3 Fibre coupler -   4 Optical wave guide -   5 Mode coupler -   6 Fibre coupler -   7 Optical wave guide -   8 Optical wave guide -   9 Mode coupler -   10 Measurement cell -   11 Photo detector -   12 Analogue/digital converter (ND converter) -   13 Controller -   14 Mode coupler -   15 Photo detector -   16 A/D converter -   18 Source controlling path -   19 Measurement path -   20 Referencing measurement cell -   22 Sensor -   30 Measurement cell -   31 Light inlet of non stabilized light source respectively disturbed     inlet path -   32 Light outlet to the photo detector -   33 (Small) glass plate -   34 Concave mirror as reflector -   35 Inlet for light back coupling -   36 Outlet of light back coupling -   37 Optional glass plate -   38 Direction of the first light path -   39 Direction of the second light path -   40 Measurement cell -   41 Collimating lens -   42 Half transparent mirror -   43 Full mirror -   44 Rotor -   45 Direction of the first part of the light -   46 Direction of the second part of the light -   47 Reflection of the second light part -   48 Direction of media flow 

1. A method for determining the concentration of substance(s) in gaseous or fluid media using optical absorption spectroscopy with broadband light sources, wherein the light of an optical fibre-coupled broadband light source is guided by way of an optical wave guide and a mode mixer after the light source directly via an optical path to a free path-absorption measurement cell, is partially absorbed therein and guided via an optical wave guide and mode mixer after the measurement cell to a measurement detector, and that the influences of the emission characteristics of the broadband light source are referenced via a reference detector which is connected via an optical wave guide coupler, a mode mixer and an optical wave guide directly to the optical fibre-coupled broadband light source, wherein the measurement detector and the reference detector may be the same.
 2. The method of claim 1, wherein for referencing the light source and/or for determination of the concentration of further substances several spectral different broadband optical fibre coupled light sources are added to the optical path via mode mixer and optical wave guide coupler, and the emission of the light sources are separated from each other at the detector and reference unit.
 3. The method of claim 1, wherein a part or the whole optical wave guide outside the measurement cell is realized via free path optics.
 4. The method of claim 1, wherein instead of a reference detector a self referencing measurement cell is used, where the path length of the interaction between the irradiated light and the measuring media is varied and thus a reference signal is generated, whereby disturbances within the emission characteristics can be suppressed.
 5. A device for determining the substance concentration in gaseous or fluid media via optical absorption spectroscopy with broadband light sources according to the method of claim 1, wherein the light of an optical fibre-coupled broadband light source (1) is guided by way of optical wave guide (2) and mode mixer (3) after the light source directly via an optical path with mode mixer (5) and optical wave guide (6) into the free path absorption measurement cell (7), it is partly absorbed therein, and is guided by way of an optical wave guide (9) and mode mixer (10) after the measurement cell (7) to a measurement detector (11), and that the influences of the emission characteristics of the broadband light source (1) are compensated via a reference detector (13) which is connected via an optical wave guide coupler (4), mode mixer (14) and optical wave guide (12) to the optical fibre-coupled broadband light source (1), wherein the measurement detector (11) and the reference detector (13) may be the same.
 6. The device of claim 5, wherein for referencing the light source and/or for determining the concentration of further substances several spectrally different broadband optical fibre coupled light sources (1A, 1B, . . . ) are added to the optical path via mode mixer and optical wave guide (19), and the emissions from the light sources are separated from each other at the detector unit and reference unit.
 7. The device of claim 5, wherein a part or the whole optical wave guide outside the measurement cell (7) is realized in free path optics.
 8. The device of claim 5, wherein instead of the reference detector a self referencing measurement cell (16) is installed, wherein the path length of the interaction (17) between the irradiated light and the media to be measured is varied and thus a reference signal is generated, whereby disturbances within the emission characteristics can be suppressed.
 9. The device of claim 8, wherein the self referencing measurement cell is realized as an inclining or rotating glass plate (33), whereby the interaction path length of the measurement media is varied.
 10. The device of claim 8, wherein at the end of the measurement cell a concave mirror is implemented as reflector.
 11. The device of claim 8, wherein the light coupled into the self referencing measurement cell (40) is partly reflected by a half mirror (42) to a full mirror (46), and an additional rotor propelled by the flow of the media and located within the optical path between the half and the full mirror (42 and 46, respectively) temporarily interrupts the reflection of the full mirror (46), and thereby the interaction path length and thus the measurement signal for referencing is varied.
 12. The device of claim 8, wherein the strength of the interaction is varied via the pressure, and the pressure is detected via a sensor (18) or defined by way of an actuator. 